1. Field of the Invention
This invention relates generally to optical sensors, and more particularly to high speed, highly sensitive, optical sensing platforms for evanescent wave surface detection applications, i.e., an evanescent interferometer biosensor.
2. Discussion of Related Art
Evanescent wave surface detection is an optical technique that has been used in various applications such as the detection of substances in liquid and gaseous samples and the measurement of certain properties of liquid and gaseous samples, including, e.g., changes in refractive indices and ionic concentrations of the samples.
The evanescent wave surface detection technique typically includes sensing a change in the local environment at the surface of a waveguide. The waveguide surface is often coated with a chemically or biologically sensitive layer, to which targets within a liquid or gaseous sample are then bound. Light is coupled into the waveguide, which, as it propagates through the waveguide, produces evanescent wave fields that reach out and penetrate the chemically or biologically sensitive layer and the sample bound thereto. Because evanescent wave fields that correspond to different spatial modes of the propagated light typically penetrate at different depths, information relating to the depths of penetration for the different spatial modes can be used to characterize the liquid or gaseous sample provided at the surface of the waveguide.
For example, the detection and/or measurement of very small numbers of microorganisms in a sample, using evanescent wave surface detection techniques, typically requires amplification, or enrichment, of the target microorganisms population before detecting and/or measuring the sample is possible. This is often accomplished using culture enrichment techniques that may take up to several days to complete. However, some evanescent wave surface detection techniques permit direct and rapid detection and/or measurement of very small numbers of target microorganisms by transferring the amplification process from the biological domain to the photonic domain.
One such evanescent wave surface detection technique uses fluorescent markers for detecting and/or measuring substances in a liquid or gaseous sample. Specifically, targets, i.e., analytes, within a sample, which are bound to the surface of a waveguide, are tagged, or labeled, with fluorescent markers. Light is coupled into the waveguide. As the light propagates through the waveguide, evanescent wave fields reach out into the tagged sample and excite the fluorescent markers. The target microorganisms in the tagged sample are then detected and/or measured by monitoring the intensity of the sample's fluorescence.
An evanescent wave surface detection technique that uses fluorescent markers, however, has some shortcomings. For example, the expense and complexity of reagents used for tagging the targets affect its utility. Still further, the process of tagging the sample with fluorescent markers has some drawbacks. Specifically, it is often difficult to ensure that only the target analytes are tagged. Frequently, however, random substances bound to the waveguide surface are tagged also, thereby affecting the detection and/or measurement of the desired targets. Such non-specific binding of random substances can adversely affect, e.g., the signal-to-noise ratio (SNR) of that evanescent wave surface detection technique.
Another optical evanescent wave surface detection technique that can be used to detect and/or measure small numbers of target analytes within a sample involves monitoring changes in the intensity of light related to the evanescent wave fields due to the bound sample. This evanescent wave surface detection technique is used in some commercially available instruments, such as the BIAcore.TM. surface plasmon resonance (SPR) instrument manufactured by Amersham Pharmacia Biotech AB, Uppsala, Sweden.
Specifically, the evanescent wave surface detection method used with an SPR instrument typically comprises coating the waveguide surface with a thin layer of metal; immobilizing "selective" receptors to the metal layer; and then capturing the sample onto the receptors. Light is coupled into the waveguide, which, as it propagates through the waveguide, causes evanescent wave fields to reach out into the sample layers on the waveguide surface. The bound targets alter the effective index of refraction (n) of the metal layer. The evanescent wave fields resonantly transfer energy to a surface plasmon, and the intensity of the evanescent wave fields is monitored at an energy matching condition.
However, an evanescent wave surface detection technique used with the SPR instrument has some shortcomings. For example, biochemical and environmental factors such as non-specific binding and temperature variation typically limit the sensitivity and stability of that evanescent wave surface detection technique. Furthermore, the BIAcore.TM. brand SPR instrument is commercially expensive; hence, it is often inappropriate for use in low-cost applications.
Still another evanescent wave surface detection technique for detecting and/or measuring untagged substances in a bound sample is disclosed in U.S. Pat. No. 5,120,131 (the "'131 patent") to Lukosz. According to that disclosed invention, a measurement sample is bound to the waveguide surface, and light is coupled into the waveguide, thereby causing evanescent wave fields to reach out into the bound sample. Specifically, light is coupled into the waveguide so that two mutually coherent, orthogonally polarized modes propagate through the waveguide simultaneously and coaxially. As a result of the interaction between the propagated light and the bound sample, the respective refractive indices of the two guided modes, i.e., the transverse electric (TE) and transverse magnetic (TM) change. Relative changes in the refractive index (n) of a measurement sample with respect to the refraction index of a reference sample can be measured with an interferometer, and those measurements can be used for characterizing the bound sample. These changes are manifest as an optical phase change of light traveling through a medium.
However, the evanescent wave surface detection technique disclosed in the '131 patent has some shortcomings. For example, this evanescent wave surface detection technique typically lacks the stability required for accurately detecting and/or measuring very small numbers of targets. This is because the stability of that evanescent wave surface detection technique typically is limited by biochemical and environmental factors, i.e., noise, such as non-specific binding and temperature variation of the bulk liquid, which often result in less than optimal SNR ("signal-to-noise ratio"). Moreover, thermal and mechanical perturbations, which are major sources of noise and which adversely affect the SNR.
It would be desirable, therefore, to provide an improved evanescent wave surface detection technique and device for detecting and/or measuring substances in liquid and gaseous samples. Such an evanescent wave surface detection technique and device would have the stability and sensitivity required for accurately and directly detecting and/or measuring low levels, e.g., as low as a single microorganism, of small molecules, bio-molecules, and/or microorganisms in a sample. Moreover, such a detection technique would have the stability and sensitivity required for accurately and directly detecting and/or measuring low levels of small microorganisms without requiring prior amplification, i.e., enrichment, of the target analyte population. It would also be desirable to have an evanescent wave surface detection technique and device for detecting and/or measuring small numbers of small molecules, biomolecules, and/or microorganisms in a sample that provides results quickly and can be implemented at relatively low cost. Furthermore, it would be desirable to have an evanescent wave surface detection technique and device that removes noise associated with thermal and mechanical perturbations to maximize the SNR.